1. Technical Field of the Invention
The present invention relates generally to a linear solenoid working to move a plunger linearly through a magnetic attraction and a solenoid-operated valve designed to move a valve body using such a linear solenoid.
2. Background Art
A typical example of conventional linear solenoids will be described below with reference to FIGS. 5(a) to 7(b).
The linear solenoid is designed to pull a plunger to a stator linearly through a magnetic attraction produced by energization of a coil. The plunger is constantly subjected directly or indirectly to a return pressure as produced by a spring. For example, Japanese Patent First Publication No. 2000-230660 discloses such a type of linear solenoid.
The linear solenoid has a magnetically attracting structure which is typically of one of three types: flat, tapered, and magnetic saturation.
The flat type is, as illustrated in FIG. 6(a), made up of the stator J1 and the plunger J2 having flat surfaces facing each other in an axial direction thereof.
The tapered type is, as illustrated in FIG. 7(a), made up of the stator J1 and the plunger J2 having tapered or curved surfaces opposed in an axial direction thereof in order to increase a magnetically attracting area of the stator J1.
The magnetic saturation type has the structure, as illustrated in FIG. 8(a), in which the stator J1 has formed in an end thereof a recess J3 into which the plunger J2 is insertable without any physical contact therewith and also has an outer peripheral edge J4 tapered to produce a variable magnetic resistance to a stroke of the plunger J2, and the plunger J2 has an outer peripheral edge J5 tapered to produce a magnetic gap changing with a change in stroke of the plunger J2.
In operation of the flat type of liner solenoid, as shown in FIG. 6(a), when the magnetic gap between the stator J1 and the plunger J2 in the axial direction thereof is small, it will produce a greater magnetic attraction acting on the plunger J2 in the axial direction. The magnitude of the magnetic attraction is, as illustrated in FIG. 6(b), substantially inversely proportional to a square of distance between the stator J1 and the plunger J2. Thus, when the magnetic gap between the stator J1 and the plunger J2 is great, it will result in a greatly decreased magnetic attraction acting on the plunger J2. The flat type, therefore, has a difficulty in producing a greater stroke of the plunger J2.
Usually, the magnitude of the magnetic attraction increases hyperbolically with a decrease in the magnetic gap between the stator J1 and the plunger J2. Therefore, when the magnetic gap decreases, it may produce an excessive magnetic attraction which gives rise to physical deformation or deterioration such as wear of operating parts of the solenoid. In order to avoid this, it is necessary to set the magnetic gap great when the plunger J2 reaches the end of the stroke thereof. This, however, results in a difficulty in decreasing the size of the coil in order to ensure a desired amount of magnetic attraction when the plunger J2 starts to move.
In operation of the tapered type of linear solenoid, as shown in FIG. 7(a), when the magnetic gap between the stator J1 and the plunger J2 in the axial direction thereof is small, it will produce a magnetic attraction acting on the plunger J2 in the axial direction which is greater than that produced by the flat type of linear solenoid because the curved surfaces are formed on the opposed ends of the stator J1 and the plunger J2. However, when the magnetic gap between the stator J1 and the plunger J2 is great, it will, like the flat type, result in a greatly decreased magnetic attraction acting on the plunger J2. The tapered type, therefore, has also a difficulty in producing a greater stroke of the plunger J2.
Like the flat type, the magnitude of the magnetic attraction increases, as illustrated in FIG. 7(b), hyperbolically with a decrease in the magnetic gap between the stator J1 and the plunger J2. Therefore, when the magnetic gap decreases, it may produce an excessive magnetic attraction which gives rise to physical deformation or deterioration such as wear of operating parts of the solenoid. In order to avoid this, it is necessary to set the magnetic gap great when the plunger J2 reaches the end of the stroke thereof. This, however, results in a difficulty in decreasing the size of the coil in order to ensure a desired amount of magnetic attraction when the plunger J2 starts to move.
In operation of the magnetic saturation type of linear solenoid, as shown in FIG. 8(a), when the plunger J2 enters the recess J3 of the stator J1, it will produce a magnetic flux oriented in a radius direction of the stator J1, thereby reducing a change in the magnetic attraction acting in the axial direction of the plunger J1 with a change in stroke of the plunger J2 and permitting an effective stroke of the plunger J2 to be increased. The outer tapered edges J4 and J5 work to produce a constant change in magnetic attraction acting on the plunger J2 in the axial direction thereof with a change in stroke of the plunger J2.
However, when the plunger J2 starts to move, in other words, when the magnetic gap between the plunger J2 and the stator J1 is great, the magnetic attraction acting on the plunger J2 in the axial direction thereof is small because of small areas of the opposed ends of the stator J1 and the plunger J2.
Additionally, when the plunger J2 reaches the end of the stroke thereof, that is, when the magnetic gap between the plunger J2 and the stator J1 is small, the areas of the opposed ends of the stator J1 and the plunger J2 are small, so that the magnetic attraction acting on the plunger J2 in the axial direction thereof will be small. Specifically, the magnetic saturation type of linear solenoid has a difficulty in producing a great amount of magnetic attraction directed in the axial direction of the plunger J2 during a duration between the start and end of the stroke of the plunger J2. This results in a difficulty in decreasing the size of the coil of the solenoid.
A broken line R in FIGS. 6(b), 7(b), and 8(b), indicates a return pressure (e.g., the load of a return spring) acting on the plunger J2. An arrow I in FIGS. 6(b) and 7(b) indicates an allowable range of the magnitude of magnetic attraction required to ensure the durability of operating parts of the solenoid. A hatched area in FIGS. 6(b) to 8(b) indicates the performance of the solenoid. The greater the hatched area is, the more excellent the performance is.